In electrostatic printing it is generally required to render visible or develop a latent image defined by electrostatic charges contained on a dielectric or insulative surface of a recording member. Such recording member may be a photoconductor as used in the commonly known process of electrophotography or xerography, or a dielectric material as used in facsimile recording or computer printout and the like. The term electrostatic printing also applies to other methods of latent electrostatic image formation and rendering visible same, such as for instance to those methods where a latent electrostatic image is formed on a surface by the so-called Dember effect, of by thermal means, or by physical means such as pressure or impact and the like.
The electrostatic latent images thus formed are rendered visible or developed in accordance with prior art practices by the application to the surface of electroscopic marking particles which are more or less selectively attracted to or repelled by the electrostatic charges defining the latent image, depending whether a direct or a reversal reproduction is desired. In the case of direct reproduction the electroscopic marking particles are deposited in the latent image areas, whereas in the case of reversal re-production the particles are deposited outside the latent image areas.
Such prior art methods of development fall into two distinct categories, namely the so-called dry development method and the so-called liquid development method. Both these methods are widely known.
Dry development involves the attraction of electroscopic marking particles or so-called dry toners to the surface bearing the electrostatic latent image, such electroscopic marking particles being applied in the form of a powder cloud or carried on a triboelectrically different carrier particle.
In liquid development the surface containing the electrostatic latent image is contacted with a so-called liquid toner which comprises a dispersion of electroscopic marking particles in an insulating carrier liquid having a volume resistivity in excess of 10.sup.9 ohm-cm and a dielectric constant less than 3.0. The electroscopic marking particles or toner particles are attracted from said carrier liquid to the electrostatic latent image and deposited on the surface containing said image. The electroscopic marking particles usually are comprised of pigment as coloring matter and resins or varnishes or oils which serve as dispersing aids, and fixing agents and can also confer the desired polarity and charge or sensitivity onto said particles.
In both dry and liquid methods of development the image formed on the surface of the recording member can be fixed thereon or transferred onto another surface if so desired.
Prior art toners and toner applicators, although very effective in many instances, suffer from inherent disadvantages. Thus dry toners are generally of limited resolution owing to the relatively large size of the toner particles, and in addition are only capable of achieving large area fill-in when complicated developing techniques are used. Further dry toners are generally not self-fixing and are commonly fixed by fusion after image development, which requires the application of considerable heat to melt the particles.
Liquid toners are generally applied to the recording member by a dip and squeegee system as commonly practised for instance in electrostatic office copiers employing as the recording member paper coated on one side thereof with a photoconductive coating comprising Zinc Oxide in an insulating binder. Biasing means are often used in such toner applicators to enhance image development. However due to the nature of the liquid toners and of the applicators used for same this method suffers from the disadvantage that the surface to be developed must be wetted with the toner dispersion and in most cases both sides of the recording member are wetted. Accordingly it is necessary to use a squeegee system in most cases in combination with the application of heat or a stream of warm or hot air to remove the liquid carried out by the recording member in order that the emerging copy may be sufficiently dry and the image fixed for handling purposes. This results in evaporation of the carried out liquid into the atmosphere, causing pollution of the environment with hydrocarbon vapor, and in addition a considerable quantity of carrier liquid is wasted in this manner.
Liquid toners as composed and as applied in the prior art suffer from the further disadvantage that they are relatively slow in developing action and thus place a limit on recording speed when attainment of a certain image quality is required.
Furthermore liquid toners containing a plurality of components such as polarity control agents, dispersing aids and fixing materials are generally of limited shelf life, due to chemical reaction between the components or to component aging or polymerisation or settling and the like.
It is also known that mainly due to the non-homogeneous particle size and particle charge of liquid toners it is not readily possible to develop continuous tone images, that is to say subject matter requiring true grey scale reproduction.